Provide planner option to switch gas only when a stop is required

[Rick Walsh]

These patches add the planner option to postpone a gas switch to the next
required stop.  I haven't managed to make it do silly things, but please do
check when you get a chance - no rush apply.

The option is ignored when the current gas is hypoxic.

It's interesting to see what effect delaying the gas switch has until the
next stop has on the calculated profile.  It doesn't make much difference.
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